Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
o c~ ~z~ 1-
1156565
.. . .
IMPROVED HOLLOW FI~ER SEPARATORY ELEMENT
AND METHOD OF MAK~ NG SAME
BACKGROUND OF THE INVENTION
This invention relates to improvements in hollow
semipermeable fiber elements of the type disclosed in Mahon
U;S. Patent 3,228,876. Such elements are adapted for use in
industrial osmosis, ultrafiltration or dialysis processes,
and are particularly useful in medical applications including
blood oxygenation, and purification by hemodialysis or
hemofiltration.
Mahon type semipermeable fiber elements have achieved
their greatest commercial utilization in artificial kidney
devices having the general confiauration of a tube and shell
heat exchanger similar to that shown in U.S. Patent 3,228,377.
Such devices couple, or seal, the hollow fiber bundle into
a tubular shell by using a solidified castable resin tube-
sheet on each end of the fiber bundle and sealing the outer
rim of the tubesheet to the inner wall of the shell. The
outer end of the tubesheet is transversely severed to form
an outer end planar surface which exposes the open fiber
ends, and that planar surface then becomes the inner wall
of an outwardly extending contiguous blood chamber.
Tubesheets for artificial kidneys are typically formed
by centrifugally casting a castable synthetic resin around
the fiber end portions while the fiber bundle is positioned
within the shell such that the casting resin solidifies
within the end portions of the shell and concurrently adheres
to and seals the rim portion of the solid tubesheet to
the inner walls of the shell. This procedure has been
successful for combinations of certain resin shells and
~.
2.
certain castable resins adhesive to those shells, notably
thermosetting resins of the epoxy and polyurethane type
as disclosed in U.S. Patents 3,619,459, 3,703,962; and
3,962,094. Prior to this invent;on, however, to our
knowledge, no one has successfully employed thermoplastic
resins in a process of centrifugally casting tubesheets
in the manufàcturë of~ artificial kidneys. The only kno~n
use of thermplastic resins in arti~icial kidney tubesheet
manufacture is that disclosed in Tigner V.S. Patent 4,138,460.
It has long been recognized that thermoplastic resins
offer advantages as tubesheets for hollow fibers that are
not possessed by thermosetting resins. Thermoplastic
resins offe~ faster potting cycle times than thermosetting
resins and thereby reduce the overall production time re-
quired to manufacture an artificial kidney ready for testing;
thermoplastic resins are free from noxious vapor or gas
generation during casting which may occur with certain
epoxy and polyurethane resins; thermoplastic resins are
cheaper than thermosets and thermoplastic tubesheet-shell
devices can be ethylene oxide sterilized easier and in
less time than corresponding thermoset tubesheet-shell
devices.
On the other hand thermoplastic resins have entirely
different handling characteristics than thermosetting
tubesheet resins and these characteristics have, prior
to this invention, prevented successful adoption of thermo-
plastics for centrifugal casting of hollow fiber tubesheets.
The handling characteristics referred to stem from the
basic reaction of thermoplastics to temperature cha~ges
and this reaction creates formidable problems when combined
with the necessity to penetrate between and to wet the
external wall surfaces of thousands of capillary size
hollow fibers and thereafter to solidify into a sound,
internal-void-free tubesheet. Epoxy and polyurethane
resins or polymers, become polymers bv chemical reaction
between initially fluid multi-components, or comonomers,
and generate heat during reaction; the generated heat causes
the reaction product to solidify, or set, at or above
some high threshold temperature. In contrast, thermo-
plastic resins are polymers that are solid at room temperature
B 5 6 5 o
but soften and become liquid as the temperature rises pasta threshold value; molten thermoplastic resins then solidify
as the temperature is reduced and passes through the threshold
temperature on the way back down toward room temperature.
Thermoplastics also have a substantially greater shrinkage
dur~ng solidification than thermosetting resins. Thus,
translating these thermoplastic resin characteristics into
required ~andling conditions during centrifugal casting, it is
necessary to, first, raise the temperature of the selected
ther~oplastic~resin to convert it into a liquid, preferably
... . . .
a low viscosity liquid, and during casting to control the
temperature of the entire molten mass of resin so as to insure
~. ... . .
penetr~tion into and around each fiber in the bundle; the con-
dition tha;t must be avoided is localized temperature drop below
the solidification threshold temperature and a resultant
flow blockage anywhere in the inflow path of the resin
prior to the arrival of liquid resin at the furthermost
- contemplated point from the infeed location. Second, the
shrinkage of the mass of the cast liquid resin confined
within a mold during solidification must be controlled so
as to counteract the resin tendency to contract from the
liauid pool toward each solidifying location in the mass.
It is also r.ecessary to recognize that the volume of thermo- ;
plastic resin shrinkage is so great that it has not been
found to be possible to solidify a disc-shaped tubesheet
within a tubular shell and retain a sound, non-fractured,
adhesion seal between the rim of the tubesheet and the inner
wall of the shell as is routinely achieved with the commercially
used thermosetting tubesheet resin compositions. This
failure of the thermoplastic tubesheet to adhere to the
tubular shell wall is serious because it necessitates
formation of a separate seal between the shell and tubesheet
in some other fashion to separate the shell into the
desired three separate fluid-tight isolated zones, e.g.,
~5 in the case of an artificial kidney, a central dialysate
zone-between two spaced apart end blood cham~ers.
In artificial kidneys employing thermosetting resin
tubesheets it is conventional to form the blood chambers by
sealing à generally cup-shaped member against the planar
15~5 (--
outer end surface of the tubesheet by a conventional circular
O-ring as illustrated in FIG. 4 of U.S. Patent 3,882,024.
With thermoplastic tubesheets, such blood chamber constructions
are not feasible due to inability to form an effective seal
between the thermoplastic tubesheet and the inner shell wall.
Prior to this invention all of the problems above
identified ~ave remained unsolved.
It is therefore the principal object of this invention
to provide an improved hollow fiber semipermeable membrane
separatory element having an integral tubesheet on each end
of a solidi~ied castable resin; each tubesheet has a solid,
axially extending disc section terminating in an outer end
planar surface exposing the open ends of the fibers therein,
and a radially o~twardly tapering surface extending from
the inner end surface toward the outer portion thereof; the
tubesheet optionally includes a second tapered portion that
extends outwardly from the outer face of the disc portion
and its peripheral surfaçe tapers radially inwardly toward
the outer end planar surface which exposes the open ends ;
of the fibers therein.
A second important object is to provide a separatory
device which incorporates the new separatory element of this
invention and provides improved means for sealing that element
into ~ surrounding shell, or jacket.
Another obJect of this invention is to provide a method
suitable for centrifugally casting thermoplastic castable
resin tubesheets on each end of a bundle of hollow semiperm-
eable fibers to form the new separatory element of this
invention.
SUMM~RY OF THE INVENTION
The improved hollow fiber semipermeable membrane
separatory element comprises a bundle of continuously hollow
semipermeable fibers having a solidified castable resin
tubesheet on each terminal portion which ~oins the exteriors
of the fibers to each other in a solid tubesheet. The
tubesheet has an axially extending disc portion which
terminates in an outer end planar surface exposing the
open ends of the fibers therein. On its opposite, or
inner end, the disc portion tapers from the inner end
axially and radially outwardly for at least a portion
658~ C)
. - ~ 4A.
of the length of the axial extent of the inner portion
of the disc section of the tubesheet. The tubesheet may
include a frusto-tapered outwardly extending portion, the
periphery of which tapers radially inwardly from the disc
section and the outer end of the frustum becomes the
outer planar surface of the tubesheet which exposes the
open fiber ends; the hollow fibers are confined to the
axial center portions and extend through both the disc
portion and the frusto-tapered portion and are oriented
generally parallel, or spirally around the axis; so that
the disc portion includes an outer annular rim free of fibers
and the fibers extend substantially to the outer edge, or
periphery, of the frusto-tapered portion, at least at tùe
: ` C) s. ll~B565 (
outer end planar surface of the frustum.
- The improved sealing means between a surrounding jacket,
or shell, and the peripheral surface of a tubesheet consists
of a radially inwardly extending tapered section on the
inner wall of the jacket, or shell, the seal resulting from
pressure urging the peripheral tubesheet wall axially inwardly
to effect a liquid and gas tight sealing of the tubesheet
wall against the tapered shell, or jacket, surface.
The improved method of centrifugally casting thermoplastic
resin tubesheets on each end of a bundle of hollow fibers
adds to conventional centrifugal casting process steps the
concurrent temperature control of the fibers relative to
the temperature of the molten thermoplastic resin during
centrifugal rotation to achieve fiber penetration and temp-
erature control of the cast resin mass so as to cause pro-
gressive solidification in the direction from the outer edge
surfaces and the outer end portion of the cast resin axially
and radially inwardly in said frusto-tapered portion and
thereafter into the adjacent disc portion with the last
portion to solidify being the axial center portion of the
disc.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side view of one embodiment of the im-
proved separatory element of this invention;
2S FIG. 2 is an exploded schematic view showing the parts
used in the centrifugal casting process prior to assembly
and including, from right to left, the jacket containing
the tapered cavity, the outwardly extending fiber bundle,
and the end mold;
FIG. 3 is a cross sectional view of the parts assembled
in the end mold preparatory to centrifugal casting;
FIG. 4 is a fragmentary cross section similar to FIG.
3 showing the solidified tubesheet after cooling and shrinkage
from adjacent molding surfaces;
FIG. S is a cross section showing the new element of this
invention in sealed assembly in the jacket in which centrifugal
casting of the tubesheets was effected; and
FIG. 6 is a fragmentary enlarged cross sectional
view showing the sealing means in the lower wall section
Of the device of FIG. 5.
6 . 1 1 ~ 6 5 6 5
DETAILED DESCRIPTION OF THE INVENTION
The improved separatory element of this invention
as illustrated by the embodiment thereof generally designated
10 and shown in FIG 1 comprises a bundle of fibers generally
designated 12 having their terminal portions secured together
in tubesheets generally desi~nated 14 and 16. Tubesheets 14
and 16, which are of similar confiauration and construction,
consist of a disc-shaped portion 18 and 19 and a frusto-
tapered portion 20 and 21, as shown.
The fibers 13 in bundle 12 are continuously hollow
semipermeable, fine, capillary size fibers having selected
permeability tQ best provide the desired separation such
as removal of solute from liquids as in ultrafiltration or
dialysis, the re~oval of water or other fluid portions and
dissolved solutes from blood as in hemofiltration, or the
introduction of oxygen into blood as in blood oxygenation.
Such fibers are generally of the type disclosed in U.S.
Patents 3,228,876; 3,423,491 and 3,532,527 when the separation
involves ultrafiltration or dialysis, particularly hemo-
dialysis, and include cellulose made by deacetylating
,, , _ , _ ........... . . . . . . t
cellulose acetate as taught in U.S. Patent 3,546,209 or -
cellulose made by the cuprammonium process, or cellulose
acetate, or other cellulose esters, or polyesters, or
polyamides; for hemofiltration the fibers may be any
protein-retentive semipermeable fiber such as cellulose
acetate or anisotropic fibers of the type disclosed in
U.S. Patent 3,615,029; fibers for oxygenating blood may be
of the polyorganosilaxane type disclosed in U.S. Patent
3,798,185.
Fiber bundle 12 includes a plurality of individual
fibers 13 which extend between tubesheets 14 and 16 and
the open end of each fiber terminates in the outer end planar
surface 22 and 23 of frusto-tapered portions 20 and 21,
respectively. The numbers of fibers 13 in a bundle 12
varies widely depending on contemplated end use, for example
between about 3,000 and 30,000, and for medical applications
normally varies between about 5,000 and about 20,000. Fiber
size is also variable but typically is in the range of about
50 to about 400 microns internal diameter with a wall
thickness in the range of about 10 to about 80 microns.
1156565
7.
Tubesheets 14, 16 as shown in FIG. 1 repr~sent the
preferred embodiment of the ~ew element of this invention
and disc portions 18 and 19 thereof are shown as circular
in cross section, and frusto-tapered portions 20 and 21 are
shown as frustums of a cone; but it is to be understood
that their shapes are illustrative only and that other
shapes are satisfactory and benefit from the improved
features of the preferred embodiment of FIG. 1. Equally
useful shapes for tubesheets 18, 19 include lenticular,
elliptical or combinations of other curvilinear or flat
wall portion or sections which collectively define any of
the cross sectional shapes illustrated in FIG 3 and FIGS.
6-10 inclusive of Tigner U.S. Patent 4,138,460, or the
like; it will be further appreciated that frusto-tapered
portions 20, 21, in each instance, project outwardly from
tubesheets 13 and 19 and radially inwardly to form an outer
planar surface of the frustum having a similar cross
sectional shape to that of the disc portion with which it is
integral. In terms of difference from heretofore known
centrifugally cast tubesheets for hollow fiber devices the
outwardly projecting frusto-tapered portions 20, 21 are
new and serve two important new functions. First, portions
20, 21 gather, or confine, all of the hollow fibers 13
into the radially inwardly tapering portions such that
open fiber ends cover substantially entirely the outer
end planar surfaces 22, 23. Second, the tapered peri-
peral wall surface portions 20, 21 provide sealing surface
area 24 for pressure sealing engagement with a cup-shaped
correspondingly tapered blood chamber forming means
generally designated 26, as shown in FIG. 5. The wall 28
of means 26 seals against portions 20, 21 to thereby form
blood cavity 30. Fibers 13 located adjacent to the edge
areas of planar surfaces 22, 23 eliminates the opportunity
for blood clotting in stagnant areas which are sometimes
present in blood chambers formed by previously used pro-
cedures of sealing a header to a planar tubesheet surface
of disc portion 18, 19 by an O-ring seal of the type
shown in FIG. 4 of U.S. Patent 3,882,024. A further
advantageous result is that the tapered outer surface
r~
---" 1 15~565
8.
20, 21 permits a blood, or fluid tight seal with the sur-
rounding header 26 which is deformable thereagainst in
elongated area 24 by a combination of radial and axial
force which insures maintenance of a iluid tight seal in
case of creep and a seal independent of the type of seal
between the inner surface of the jacket and the outer
surfàce of disc portion 18, 19 as ~ill be explained in
greater detail in connection with the device shown in
FIGS. 5 and 6.
Element 10 may satisfactorily include tubesheets 14, 16
centrifugally cast from any of a large number of satisfactory
thermosetting resin compositions, for example, epoxy
resins identified in U.S. Patents 3,619,459 and 3,703,962
or polyurethane resins identified in U.S. Patents 3,962,094
.. . . . . .
and 3,708,071 and the res~1ltant elements are a part of this
invention. Copending Canadian Appln. SN 351,822, owned by
the assignee of this application and filed concurrently
with this application, describes an improved process and
molding apparatus for centrifugally casting tubesheets 14,
16 with a thermosetting resin on a bundle of cellulose fibers
which employs means and procedures that insure fibers
centered in the frusto-conical portion of the resultant
tubesheet.
The improved method of this invention is specific to
a process for making tubesheets 14, 16 by the use of
thermoplastic resins, as above stated. Since thermoplastic
resin centrifugal casting is entirely distinct and different
from thermosetting resin centrifugal casting, the requirements
as to physical porperties of the resin, fibers, compatible
materials for use as jackets and molds, and the controlling
operational parameters during casting and resin solidification
will first be described in general terms to aid in selection
of specific materials and conditions to enable one to design
specific elements for specific end uses. Satisfactory
thermoplastic resins must have low viscosity at temperatures
in the range of about 100C to about 150C and in one
preferred embodiment using cellulose acetate fibers and
a polypropylene jacket the resins having a viscosity less
than about 1500 centipoises at 135C were preferred. Such
resin achieved penetration between the fine, capillary fibers
- 1156~65
9.
without causing fiber wall collapse under the pressure of
casting; the resins must wet and adhere to the fiber
outer wall surfaces and must be non-adherent to the jacket,
or shell, which contains the fiber bundle 12 during casting;
the resins must solidify to solid form without voids, cracks
or fractures in the cast shape and must possess resistance
to loss of hardness tensile and compressive strength at
temperatures normally encountered during shipment, storage
or use, for example, up to temperatures of about 160F.
Additionally, the resin must be insufficiently brittle at
room temperature to fracture or crack when transversely
severed through the tapered frustum and the contained fibers.
The presence of fibers throughout the tapered frustum, par-
ticularly those located at its peripheral edge surfaces at
the plane of the transverse cut aids in fluid distribution
and eliminating stagnant areas of fluid on that outer surface.
From the processing standpoint, in selecting fibers,
care should be taken to select fibers with resistance to
softening at temperatures approximating the melting point
2~ of the selected casting resin to thereby insure that the
fibers substantially retain their circular cross section
during casting; this is necessary because the fibers must
be preheated to the approximate casting temperature prior
to casting in order to prevent undesired resin solidification
due to cooling upon contact with cold fiber surfaces. For
elements of this invention that are to be used in artificial
kidney devices which concurrently remove water and body
poisons such as urea, creatinine, etc., during hemodialysis,
a preferred fiber is cellulose acetate made bv the process
disclosed in Kell, et al Canadian Patent No. 1,107,020. me jacket,or shell,
and endmold must be fabricated from a material which does
not adhere to the selected thermoplastic casting resin;
alternatively, all surfaces thereof which contact the
casting resin must be treated with a mold release agent,
or material, to prevent such adhesion. In contrast to
thermosetting resins which are selected for their strong
adhesion properties to the selected shell, thermoplastic
resins crack, fracture or disintegrate during shrinkage
or cooling unless they are non-adherent to all contiguous
~3 11~6565 ~ i
10 .
retaining surfaces throughout their solidification. A
preferred jacket, or shell, material which is non-adherent
to many satisfactory thermoplastic castable resins without
separate surface pretreatment is polypropylene. With a
mold release agent pretreatment, any of the commercially
used shell materials are satisfactory. Suitable mold release
agents include waxes, polytetrafluoroethylene, and the like.
Generally stated, the above enumerated resin require-
ments are satisfied by the ethylene vinyl acetate copolymers,
particularly the lower molecular weight, low viscosity
resins of this type, for example, resins selected from those
disclosed in U.S. Patents 3,428,591 and 3,440,194. Specific
identification of suitable resins and their properties are
given in the Examples.
After selection of fibers, resin and jacket the steps
of the method of this invention comprise inserting the
bundle 12 into a jacket, or shell, generally designated 32
FIG. 2, and inserting the assembled bundle and jacket into
endmold 34 in preparation for centrifugal casting. The
8,000 to 10,000 fibers in bundle 12 may be formed on a
conventional beltwinder by known procedures of the type
disclosed in U.S. Patent 3,755,034, or equivalent and the
resulting bundle terminates at its ends in a plurality of
annular layers 17, which are secured by band means 15,
illustrated as a tape such as nylon or polypropylene.
Endmold 34 is especially fabricated to enable necessary
temperature control of the molten thermoplastic resin
during casting and the cooling of that resin to form
void-free, sound tubesheets 14, 16 which surround and
support fibers 13 for substantially the full axial length
thereof. As seen in FIG. 3, endmold 34 includes an outer
end metallic section 36 for receiving the band end 15 of
bundle 12. Bundle 12 is positioned therein such that
inwardly projecting pin 38 penetrates aperture 40 located
on the longitudinal axis of bundle 12. Both the outer
endmold section 36 and pin 38 are fabricated from a material
having high heat conductance, for example, aluminum, copper,
brass or bronze to facilitate heat removal from the molten
resin centrifugally cast into its cavity 40. Endmold 34
also includes a second cavity defining section 42 which
. . .
,
J 11~5~5 (`
11 .
surrounds the inner end 37 of metallic section 36, and mold
section 42 is fabricated from a material having low heat
conductance, for example, polycarbonate, or glass fiber
filled polypropylene, epoxy or impact polystyrene. The
inner portion 44 of mold section 42 is surrounded by a
thick layer of insulation 46 to further retard heat loss
from its cavities. Mold section 42 is provided with an
inner surface tapered wall 48 which defines the angle
of taper on the outer surface 20 of the resultant frusto-
tapered portion 25 of the cast tubesheet, FIG 5. Innerwall 44 defines a cavity 50 which receives the outer wall
52 of jacket 32. Jacket 32 is provided with an axially
and radially outwardly extending tapered surface 54 which
abuts the radially outwardly extending flange portion 56
to define a cavity 58, which in turn determines the shape
ana size of disc portion 60 of the resultant tubesheet
having peripheral tapered sealing surfaces 18, 19.
The assembly of endmolds 34 on each end of shell 32 to
the positions shown in FIG. 3 are then preheated to approxi-
?Q mately~the~_meltin~ point of the selected thermoplastic resin,usually in the range of about 100C-140C. Similarly, the
thermoplastic resin, the resin container and resin delivery
means connecting the resin container and the inlet ports in
each end of jacket 32 are heated to a slightly higher temp-
erature than the melting temperature; for example, about 135C.
The heated assembly and the heated resin containerare then placed into the carriage of a centrifugal casting
apparatus of conventional type such as that shown in U.S.
Patent 2,442,002 such that the resin delivery tubes in-
terconnect the inlet ports of the jacket 32 and the resincontainer. The centrifuge is started and the speed is
raised to selected speed in the range of about 900 to 1400
rpm, and spins the fiber bundle assembly for approximately
25 to 35 minutes. The assembly spins in a horizontal
plane and in the apparatus used to produce the elements
shown in FIG. 1, the outer end 36 of endmold 34 was i
approximately 5 inches from the axis of rotation. ~l
After expiration of the normal approximately 2 to S ';
minutes for resin penetration between and around fibers 13
and into and filling cavities 58 and 40, cooling air is
~ `
~_! 115~565
blown over the rotating centrifuge carriage to aid resin
solidification, and after approximately 25 to 35 minutes
the centrifuge is stopped and the potted element assembly
is removed, and jacket 32 containing solidified tubesheets
is disengaged from endmold 34. After cooling the outer
ends of the tubesheets are transversely severed to produce
frusto-tapered portions 25 which expose the open fiber
ends at outer planar surfaces 22, 23.
During cooling, the resin in cavities 58, 40 shrinks
radially inwardly from all of the inner walls of endmold
34 and shell 32 which define those cavities. This shrinkage
separation may be seen most clearly by comparing FIG. 3 with
FIG. 4. FIG. 3 shows bundle 12 and shell 32 in endmold
34 immediately prior to casting the liquid thermoplastic
resin into cavities 58, 40. Fragmentary exploded FIG. 4
shows a portion of the inner wall of cavity 58 after solidi-
fication of the cast tubesheet, particularly the tapered
inner end portions 18 and integral outer frusto-tapered
portion 20 which are separated from adjacent molding t
surfaces 54 and 48, respectively by shrinkage slot 62.
_ . , . ,_ _ _ . _~ . _ ....... _ _ . . . , . . _ . . . _
Slot 62 may very slightly in width in its circumferential
extension around the peripheral edges of the tubesheet 14, - j
but it is substantially uniform. The resultant tubesheet
has the external shape of the interior molding surfaces
but its solidified volume is about 8% to 15% less than
the mold~ volume depending upon the particular thermo-
plastic resin selected for use. It will therefore ~e
apparent that while the tapered peripheral surface 18 of
solidified tubesheet 14 is parallel to inner tapered wall
54 of shell 32, against which it was in contact in its
molten condition, the diameter of taper 18 on the solidified
tubesheet is less than the diameter of shell wall 54.
As shown in FIGS. 5 and 6 tapered peripheral surface
18 is mechanically sealed against shell wall 54 to form
a fluid tight seal between blood cavity 30 and dialysate
cavity 64 as well as from the outside atmosphere. The
sealing surfaces 18 and 54 are in engagement for an axial .
length extending from the inner end surface 66 to its outer
end surface 68 of disc portion 60 of tubesheet 14. This
sealing engagement results from force exerted against
B~65
. - 13.
tubesheet 14 axially inwardly in sufficient amount to move
. tubesheet 60 axially inwardly from its as-cast position,
represented by dotted line 70, to its sealed position
wherein outer surface 68 is spaced inwardly from the outer
5. end wall 72 of shell 32 for a distance 74 along taper 54.
In the preferred embodiment of the tubesheets 14, 16
which include an outwardly extending portion 20 which is
tapered radially inwardly, as shown, the sealing force
which causes tubesheet 60 to move axially inwardly into
shell 32 is obtained by pressure engagement between taper
- 20 on the peripheral surface of frusto-conical portion 25
and the tapered inner wall 28 of blood chamber forming
means, or header 26. Header 26 is provided with a radially
extending flange 76 at the inner end of wall 28 which is
engaged by lower wall surface 78 of screw band sealing
means 80. As screw band 80 is tightened, or moved axially
inwardly along upper wall 86 of shell 32 by engagement of
grooves 82 in band 80 with threads 84 on wall 86, a combination
of axial and radial pressure is exerted on taper 20 by
surrounding tapered wall 28 of header 26. The resiliency,
,, , _, _ _ _, _ . _, . _ . , , _ _ _ . _ _ . _ _, , . _ . _ , _ , _ .
or distortability of wall 28 causes a pressure sealing
engagement to occur over the area 24 and this elongated -
area of contact insures continued maintenance of a fluid
tiqht seal even under pressures sufficient to cause creep
2S of the thermoplastic resin in tubesheet portions 25 or 60.
It will be appreciated that tubesheet 60 will form an
effective seal between mating tapered surfaces 18 and 54
irrespective of the particular means used to apply the
needed axial inward pressure, or force, to cause sealing
and that tapered frusto-conical portion 25 may satisfactorily
have a peripheral wall taper other than conical up to and
including outer walls parallel to the longitudinal
axis of shell 32. For tubesheets having an outwardly
protruding portion 25 with peripheral surfaces parallel
to the longitudinal axis of shell 32 axial force can be
applied directly against the outer planar surface which
exposes the open fiber ends and for this tubesheet confi-
guration a blood chamber can be formed with a conventional
cup-shaped header sealed to the outer planar surface by
an 0-ring.
.
r~ 11~8565 (~
~ 14.
As shown in FIG. 1 the inner end taper on peripheral
surface 18 extends its full axial length at a single angle
from the axis of bundle 12, and that the internal taper
54 in shell 32 likewise extends to the outer end 72 of the
shell wall 86, FIG. 6; it will be appreciated however, that
satisfactory sealing is obtained when surfaces 18 and 54
have the same taper for only a portion of the length of.
those tapered surfaces which is adjacent to the axial
inner end 66 of disc portion 60. In such a construction,
the outer axial portions of the peripheral surface 18 may
assume any an~le that is desired so lonq as it is closer
to parallelism with the lonqitudinal axis up to and
including parallel. It will be appreciated that disc 60
will solidify to the same taper angle as the angle on
molding surface 54 adjacent the outer end 72 of upper shell
wall 86 which is employed in the centrifugal casting step,
FIG. 3; in this case, taper 54 will extend only for a portion
- of its total length, as shown in FIG 6 and the taper will
extend axially and radially inwardly only from a point
2Q spaced inwardly~from end surface 72 of shell 32. _ _
EXAMPLE I
A separatory element as shown in FIG. 1 was made by using
a bundle containing approximately 8,000 cellulose acetate
fibers approximately 10 inches long, terminating in banded
end portions as shown in FIG. 2. A polycarbonate jacket
approximately 7.25 inches long and having the internal
tapered wall configuration shown in FIG. 3, was treated
on its wall surfaces with a thin layer of polytetra-
fluoroethylene and after assembly of the fiber bundle
and jacket into an endmold shown in FIG. 3 the assembly
was heated for approximately 2 hours in an oven at 100C.
Concurrently the selected thermoplastic resin of the
ethylene vinyl acetate copolymer type, modified as
described below, and the resin reservoir and resin delivery
tubes were heated to about 135 C in an oven in approximately
2 hours. The resin composition was a mixture, in percent
by weight, of: 29~ ethylene-vinyl acetate copolymer con-
taining 17.5-18.5% vinyl acetate and available commerically
from DuPont under the trademark Elvax 410; 9% of ethylene-
vinyl acetate-acid terpolymer containing 24-26% vinyl acetate
~r~ 1156~65
15.
and commercially available from DuPont under the trademark
Elvax 4320; 42~ microcrystalline polywax, understood to
be a homopolymer of ethylene having a molecular weight of
approximately'700 and available commercially under the
trademark BARECO 655 from Petrolite Corporation; and 20%
of a polyterpene tackifier with a soltening point of 115C
and available commercially from Hercules, Incorporated .
under the trademark Piccolyte A 115. This composition
had a viscosity of about 1400 cps at 130C and about 1250
cps at 135C.
The heated endmold-jacket assembly and resin
reservoir were positioned in the centrifuge carriage to
horizontally spin the approximately 10" long assembly
at a rotation speed between 1,000 and 1,200 rpm. After
about 5 minutes, an air blower supplied room temperature
air into the circulating centrifuge and after 30 minutes
the centrifuge was stopped and the cast tubesheets in the
jacket were removed from the endmolds and severed trans-
versely slightly inwardly from the inner end surface of
the bands.
~ . , . . , . . ~
The resulting element was sealed into the jacket
using the header and screw band to axially force tubesheet
60 into sealing engagement with ~acket wall 54 and simul-
taneously form blood chamber 30.
A number of different thermoplastic resins have been
successfully used in the method of this invention and all
have been modified ethylene-vinyl acetate copolymer based
compositions. As a general ~uide to selecting specific
compositions it is desirable to use a single ethylene-vinyl
acetate copolymer, or a mixture of same, having a composite
melt index in the range of 300-400 as determined by the
method of ASTM D 1238, and further desirable to avoid use of
eth~lene-vinyl acetate copolymers having a shore hardness
less than about 72 as determined by the method of ASTM D
2240; additionally the softening point of the ethylene-
vinyl-acetate copolymer is preferably in the range of
190F - 210F as determined bv the method of ASTM E 2B.
The copolymer viscosity is substantially reduced
to enable fiber penetration by modification to include
a microcrystalline polywax of the ethylene homopolymer
(~ 565
type and the amount employed can be varied upwardly in
the range of about 20% to about 60~ of the total composition,
as needed, for centrifugal casting temperatures below about
110C to accommodate fibers which cannot withstand preheating
above about loo&, and similarly the amount needed can be
adjusted downwardly where higher casting temperatures can
be employed. Resins that are suitable may have a viscosity
as low as 100 centipoises and as hiah as 5,000 centipoises
at a temperature of 150C or less. The use of the higher
~iscosity resins requires hiqher quantities of the micro-
crystalline wax component to insure fiber penetration.
A further componènt to improve adhesion of the resin
and fiber, is satisfactorily any of the polyterpene resins
derived from alpha-pinene having softening points above
about 100C-110C and the composition may contain about
15% to 30% of the total composition of such resins.
EXAMPLE II
Another element of the FIG. 1 type was made using
the same ~acket steps and apparatus recited above in
Example I but employed a bundle of cellulose fibers made
by the process of deacetylating cellulose acetate disclosed
in U.S. Patent 3,546,209. The endmold, jacket and fibers
were preheated to 140C and the selected resin, resin pot
and resin delivery tubes were preheated to 170C. The
resin composition was a mixture, in weight percent, of
34% Bareco 655, 29% Piccolyte A 115, 29% Elvax 4320 and
8% Elvax 150. This resin composition had a viscosity
at 170~C of ab _t 1,000-1,100 centipoises.
